1. Technical Field
The present invention relates to a magneto-optical recording medium capable of amplifying output signals by enlarging recorded magnetic domains upon reading, and a reading method.
2. Background Technique
In a magneto-optical recording medium, a magnetic thin film thereof is locally heated by laser beam irradiation or other light irradiation to decrease its coercive force. Then, an external magnetic field is applied to the magnetic thin film to invert the direction of magnetization of the irradiated spots or maintain the initial magnetization direction, thereby forming magnetic domains (recorded marks). The direction of magnetization of the magnetic domains is read out by the Kerr or Faraday effect.
In an ordinary magneto-optical recording medium, the available density of readable magnetic domains is limited by the spot diameter of the laser beam used for reading, and so it is impossible to read magnetic domains having a diameter smaller than that the half of the laser beam spot diameter.
For a magneto-optical recording medium capable of reading magnetic domains having a diameter smaller than the half of laser beam spot diameter, for instance, JP-A 8-7350 discloses a magneto-optical recording medium capable of enlarging recorded magnetic domains. This magneto-optical recording medium comprises a three-layered recording film comprising, in order from an substrate side, a first magnetic layer, a second magnetic layer, and a third magnetic layer which are exchange coupled together. Upon reading, the substrate of the medium is irradiated with a laser beam while a reading magnetic field is applied to the recording film of the medium. The third magnetic layer holds recorded magnetic domains, which are copied by laser beam irradiation to the first, and second magnetic layers. The copied magnetic domains are enlarged in the longitudinal direction of each magnetic layer by reading magnetic field. The enlarged, copied magnetic domains are read out as is the case with a conventional magneto-optical recording medium. After the reading of the enlarged, copied magnetic domains has finished, an erasing magnetic field opposite in direction to the reading magnetic field is applied to erase off the copied magnetic domains for the next reading of adjacent recorded magnetic domains. By repetition of such a process, it is possible to read minute magnetic domains unreadable so far in the art. Furthermore, this method does not only achieve high resolution upon reading, but also enables output signal intensity to be in itself augmented because the magnetic domains are actually enlarged.
For this reading method, however, it is required to apply the reading, and erasing magnetic fields, viz., a modulated magnetic field depending on the recording density of magnetic domains. For instance, when the magnetic domains recorded by magnetic field modulation are read by the aforesaid method, the frequency of the modulated magnetic field upon reading is at least twice as high as that of the modulated magnetic field upon recording. For the application of a high-frequency modulated magnetic field, it is required to keep a magnetic head as close to the recording film as possible and, hence, make use of a magnetic head of the type that slides over the surface of the medium. For this reason, the recording film, a protective film formed on its surface, etc. should have sliding robustness. This renders it impossible to use a double-sided recording medium comprising a laminated substrate; there is no option but to use a single plate type medium having a reduced recording capacity per plate. An additional problem with the single plate type medium is that the substrate cannot be made thin because it is required to make sure of rigidity. For this reason, when a resin substrate with high birefringence is used, it is difficult to converge a laser beam and, hence, it is impossible to obtain high recording density. Furthermore, there are technical or cost-relating limits to high frequencies allowed for magnetic field-generating means. These limits in turn lead to limits to readable recording density as well as to an improvement in the transfer rate due to fast reading.
In the "scope of what is claimed", the aforesaid JP-A 8-7350 recites that reading takes place through the modulation of a magnetic field and/or reading light. That is, it is recited that to enlarge the recorded magnetic domains for reading, only the modulation of reading light is needed. In the "detailed explanation of the invention", however, the publication refers merely to a method of erasing off the magnetic domains by the application of a magnetic field opposite in direction to a reading magnetic field. In other words, there is only a disclosure to the effect that the pulse modulation of reading light is to achieve S/N improvements. In the example given therein, the magnetic domains recorded at a modulation frequency of 2.5 MHz are read by the modulation at 5 MHz of optical pulses and magnetic field with a sample clock timing. However, the erasure of magnetic domains cannot be achieved only by the modulation of optical pulses, as can be understood from the fact that a reading magnetic field of -100 Oe is applied; that is, a magnetic field of 100 Oe is applied in the reverse direction to the reading magnetic field.